The two main challenges in solar cell technology are the cost and the energy conversion efficiency of solar cells. Different materials and structures have been tested for several decades to address these challenges. As a result, the technology has evolved from the first solar cell generation, starting with crystalline silicon based p-n junctions, to the third generation, which includes organic photovoltaics (OPVs), dye sensitized solar cells (DSSCs), and perovskite (PVSK) solar cells. While the choices of the photoactive material in different devices (organic semiconductors in OPVs, dyes in DSSCs, and perovskites in PVSKs) are mainly based up on which materials provide strong light absorption and efficient charge generation, the device structures must be designed to collect the charges efficiently from the photoactive materials and transfer the charges to the device electrodes. The approach for selective collection of electrons from photoactive layers in OPVs, DSSCs, and PVSKs is to use a layer of a material, referred to as an electron transport layer (ETL), having an energy structure that can block holes but is transparent to electrons so as to enable electron transport.
The energy levels in the ETL and the energy barrier between ETL and the photoactive layer are critical to achieve high energy conversion efficiency in a device. Therefore, many different materials have been tested for use in forming ETLs in OPVs, DSSCs, and PVSKs. These materials include metal oxides, such as titanium oxide (TiO2) and zinc oxide (ZnO), and organic materials. Although, in theory, some materials should be more effective in improving device characteristics, in practice, significant improvement has not been achieved when ETL materials having matched energy levels have been used. This is mainly due to the other requirements for ETLs, such as optical transparency and high mobility of carriers. The combination of all the requirements of an ETL has limited the choice of material for ETL formation to only a few materials. For instance, TiO2 has been the dominant ETL in DSSCs for more than two decades.
From the above discussion, it can be appreciated that it would be desirable to have alternative ETLs that provide improved energy conversion efficiency.